On April 24,1995, NASA's Hubble Space Telescope (HST), widely acknowledged as the finest astronomical instrument ever built, celebrated its fifth anniversary in orbit (the HST is shown above in the payload bay of the Orbiter Endeavour as astronauts completed the first Hubble servicing and refurbishment mission in 1993). Just prior to the anniversary, the HST provided astronomers with another surprising major discovery, a new great dark spot in the cloudy northern hemisphere of the planet Neptune.

Months earlier - in June 1994 - Hubble images had disclosed that a large dark spot in Neptune's southern hemisphere, originally discovered by Voyager 2 in 1989, had mysteriously disappeared. The new dark spot found in 1995 is very similar to the feature first mapped by Voyager 2, but located in a different part of the planet. Astronomers say that the Hubble finding shows that Neptune has changed radically since 1989, indicating that Neptune has an extraordinarily dynamic environment in which the planet can look completely different in only a few weeks.

In June 1995 Hubble investigators announced that the HST had detected a vast disc of perhaps 200 million large comets orbiting beyond the orbit of Neptune. In the same month, the HST confirmed the existence of primordial helium in the universe, a discovery that supports the theory as to the creation of hydrogen and helium in the immediate aftermath of the Big Bang. These discoveries were among hundreds of important findings Hubble has contributed in the brief span of five years. With a resolving power calculated to be 10 times better than any telescope on Earth, the HST has compiled a rate of discovery that is unprecedented for any modern observatory. NASA Associate Administrator for Space Science Wesley T. Huntress Jr. has termed the telescope "truly a national scientific treasure." The Hubble is a cooperative program of NASA and the European Space Agency (ESA). Hubble program and project scientists have selected a "Top Ten" list of the observatory's leading discoveries since it became operational in 1990: * The first conclusive evidence of the existence of black holes, which have millions or billions the mass of the Sun.

* Indications that the universe might be much younger than previously thought, a finding acquired by Hubble's accurate measurement of the distance to a remote galaxy, which provided a basis for calculating the expansion rate of the universe.

* The first direct visual evidence that the universe is evolving as predicted by the Big Bang theory.

* The discovery that quasars, very distant and extremely bright objects, are even more mysterious than earlier thought, because many do not dwell in the cores of galaxies but are isolated in space.

* Indications that dark matter in the universe is more exotic than once thought, through a finding that nature does not produce enough of the extremely small Red Dwarf stars that were once considered a leading candidate for the universe's "missing mass."

* Further support of the Big Bang theory by refining estimates of the amount of deuterium in space (deuterium is an element created in the initial cosmic fireball that gave birth to the universe).

* A solution to the mystery of intergalactic clouds of hydrogen, provided by the observation that they are really gigantic halos of galaxies.

* An implication that planets - and presumably life - might be abundant in the universe, based on the discovery of discs of dust that might be embryonic planetary systems around young stars. The Hubble imaged immense walls of glowing gases which form a colorful backdrop to dozens of newborn stars, many of which have dust discs.

* A contribution of important detail and surprising findings relative to the spectacular 1994 collisions of Comet Shoemaker-Levy with the planet Jupiter.


* The revelation of dynamic weather changes on nearly all the planets, with a clarity once obtainable only by spacecraft fly-bys. Hubble is serving as an interplanetary "weather satellite;" an example of its imagery is shown at above, a spring image of Mars' northern hemisphere showing that the carbon dioxide frost around the permanent water-ice cap has sublimated and the cap has receded to a core of solid water-ice several hundred miles across. The image also shows an abundance of wispy white clouds, indicating that the atmosphere was cooler at the time of this image than on visits of space probes in the 1970s.

The Hubble Space Telescope was designed to work in orbit for 15 years. The 1993 refurbishment mission was the first of four planned; a second is scheduled for February 1997 and a third in November 1999. The largest scientific payload ever orbited, the 12 1/2 ton, 43-foot HST was developed by Lockheed Missiles and Space Company and Perkin-Elmer Corporation (now Hughes Danbury Optical Systems) under the management of Marshall Space Flight Center. When the telescope became operational in 1990, Goddard Space Flight Center took over responsibility for controlling the observatory and processing the imagery and data. The center for collection and distribution of the data is the Space Telescope Science Institute in Baltimore, Maryland, operated for NASA by the 17 - member Association of Universities for Research in Astronomy. The Hubble Space Telescope is the first of a series of spacecraft known as the Great Observatories, each of which is designed to operate in a certain range of the electromagnetic spectrum, including the full ultraviolet, infrared, x-ray and gamma ray ranges. This capability is important to astronomical science because each band of the spectrum offers a different set of clues to the origin and evolution of the universe.


The second of the Great Observatories is the Compton Gamma Ray Observatory (above). Launched in 1991 and managed by Goddard Space Flight Center, the Compton Observatory is investigating the most energetic of all forms of radiation. Like Hubble, Compton has been regularly making new discoveries, such as the 1994 finding described as "one of the most spectacular astro-physical discoveries of the decade: evidence that gamma ray bursts occur in the far reaches of the universe, bear an imprint of the universe's expansion, and occur so far away that they show relative time dilation." This finding provided additional evidence that gamma ray bursts are not limited to the Milky Way Galaxy.


The third of the Great Observatories is the Advanced X-ray Astrophysics Facility (AXAF), in development and scheduled for launch in September 1998. AXAF (above) passed a major developmental milestone in the spring of 1995: completion of the polishing and measurement of the observatory's eight mirrors, one of the project's toughest technical challenges. The job was completed four months ahead of schedule by the optics contractor, Hughes Danbury (Connecticut) Optical Systems. The spacecraft principal contractor is TRW Inc. The AXAF team also includes the Smithsonian Astrophysical Observatory and project manager Marshall Space Flight Center.

AXAF is expected to significantly improve scientific understanding of some of the most energetic and violent processes in the universe. It will provide images and spectrograms that will yield information on the temperature and composition of such objects as neutron stars, black holes, debris from supernova explosions, quasars, the centers of active galaxies, and hot gas in individual galaxies and clusters of galaxies.

The contemplated fourth Great Observatory is the Space Infrared Telescope Facility (SIRTF), which is intended to conduct advanced investigations of prime interest targets identified by earlier infrared observatories. Managed by Jet Propulsion Laboratory, SIRTF is in study status in anticipation of hardware development beginning in 1998 and orbital service in 2002.

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Another infrared observatory in development is the Stratospheric Observatory For Infrared Astronomy (SOFIA), a joint project of
NASA and the German space agency DARA. SOFIA (above) is an airborne rather than an orbital observatory. It consists of a 2.5 meter telescope mounted in a specially modified Boeing 747SP transport;
it is planned for initial operation in late 2000.
SOFIA will replace NASA's aging Kuiper Airborne Observatory, a modified telescope-equipped C-141 transport that has been in service since 1974. SOFlA's telescope will be three times the diameter and about 10 times more sensitive than the Kuiper sys-
tem. The new observatory will enable significant advances in the study of the solar system, star and planet formation, the makeup of the interstellar medium, galactic structure and evolution.